Consider a conference room in which a number of talkers, each with a separate microphone, are to speak. If all of the microphones are allowed to be active at the same time, problems associated with feedback and extraneous noise pickup arise. Each active microphone that is not actually being used picks up the amplified sounds from the microphone that is being used by the current talker. As a result, the amount of amplification available to the talker's microphone must be reduced to prevent feedback. In addition, each active microphone acts as a noise source that further reduces the quality of the audio from the microphone that is actually being used by the current talker. Finally, destructive interference from reinforced sound recirculated through the sound system from multiple open microphones (commonly known as "comb filtering") can cause serious aberrations in the system frequency response.
To avoid these problems, a number of prior art systems have been devised to activate only those microphones in which there exists some desired signal. One class of prior art system uses the sound level at each microphone to determine whether that microphone should be on or off. Some of the systems (e.g. Scrader, U.S. Pat. No. 4,090,032, Peters, U.S. Pat. No. 4,149,032, and Ponto and Martin, U.S. Pat. No. 4,374,300) compare the instantaneous signal level at any microphone to a reference. If the microphone level exceeds the reference level, the microphone is assumed to have a desired signal and is turned on. Systems of this type typically modulate the reference in some manner that is proportional to the current active signal level to prevent other microphones from coming on as a result of pickup of sound from the system loudspeakers. Often, systems of this type change the attenuation level of the microphone channel from the off state to the on state in an instantaneous manner, which can give rise to audible switching transients not harmonically related to the audio signal. In addition, a single initial value of the reference threshold may not accurately reflect the changes in ambient noise in an acoustic space under varying conditions of use.
Other types of systems (e.g. Anderson, Bevan, Schulein, and Smith, U.S. Pat. No. 4,489,442 and Dugan, U.S. Pat. No. 3,814,856) develop a reference level based on a sensing microphone of some type. In the case of Anderson et al., two directional microphones are placed back-to-back in a common housing. One of the microphones is the system microphone, while the other is the sensing microphone. The system microphone is oriented so as to preferentially pick up the desired sound, while the sensing microphone is oriented to pick up the background sounds. The signal at the system microphone must exceed the level at the sensing microphone by 9.54 dB to activate the system microphone. While this system solves the reference threshold problem, there are still drawbacks. The system again switches from the attenuated level to the unattenuated level instantaneously which leads to the difficulties discussed above. In addition, the system is restricted to the use of only specially manufactured microphones, eliminating the choice of other microphones whose characteristics might be better suited to the particular application.
The Dugan system, in contrast, uses a single sensing microphone situated in such a way so as to receive a signal that is representative of the ambient noise in the room. The effective threshold for other microphones is proportional to the instantaneous signal value at the sensing microphone. Also, rather than changing channel gain in an instantaneous manner, the shift from fully attenuated to fully on happens over a 10 dB range above threshold. This is accomplished via 2:1 expansion, i.e. for every 1 dB that the microphone signal exceeds the threshold, channel gain increases 1 dB up to a total of 10 dB gain change, at which time the channel reaches unity gain. Difficulties in the use of this system arise in finding an appropriate place for the sensing microphone that accurately reflects the ambient noise in the entire space. In addition, a localized increase in the ambient noise near the sensing microphone can prevent activation of microphones which have desired signals.
In another system (Dugan, U.S. Pat. No. 3,992,584), a comparison is made of the level at each microphone preamp to the level of the overall mixed signal. Each channel is attenuated by an amount that depends on the difference between the two levels. For instance, if one microphone were active, the level of that microphone and the level of the overall mix would be equal (i.e. a difference of 0 dB), so that the channel would not be attenuated. All other channels would be very much attenuated because of the large difference between the mix level and the level of an inactive microphone. If two microphones are active at the same level, the mix level (assuming no correlation between the two inputs) will be 3 dB higher than either active input. Thus, both microphones would be attenuated by 3 dB (and again inactive microphones would be greatly attenuated as before). The drawback to the Dugan scheme is that extraneous noise (whether recirculated signal from the sound system or other noise) at "inactive" microphones will compete with the active microphone for system gain. In this way, the "active" signal is amplitude modulated by the extraneous noise.
Broadly, it is the object of the present invention to provide an improved audio mixing system.
It is a further object of the present invention to provide a mixing system that does not instantaneously switch a microphone from the attenuated level to the unattenuated level in response to level changes on the microphone.
It is a still further object of the present invention to provide a mixing system that is not limited to one type of microphone.
It is yet another object of the present invention to provide a mixing system that does not depend on separate sensing microphones to determine the background noise level.
It is a still further object of the present invention to provide a mixing system in which the active signal is not amplitude modulated by extraneous noise inputted through inactive microphones.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.